The present invention relates to a feedback circuit, being applicable to, for example, a digital amplifier, etc.
In the recent years, digitalization of power amplifiers (hereinafter, amplifiers) has been quickly advanced. In particular, digitalization is remarkably accepted in the field of an audio-amplifier. Apparatuses of installing such the digital amplifier (i.e., so-called switching amplifier) include a DVD player, a mini-component, a television receiver, a personal computer, a mobile telephone, etc.
With such the digital amplifier, since a digital waveform in a rectangular shape can be obtained therefrom, including distortion of a high-frequency region through switching elements, which are on-off controlled, complementarily, the digital waveform must be converted into an analogue waveform to be supplied to a speaker, etc., for example, by removing such the high-frequency distortion therefrom. This conversion is achieved by use of a low-pass filter (LPF), for example.
Conventionally, a filter of a second order of LC or higher, including a coil and a capacitor, is applied as the low-pass filter.
By the way, the above-mentioned low-pass filter may have various kinds of adverse effects, such as fluctuation on the frequency characteristics thereof due to an impedance with respect to a load, for example, the speaker mentioned above; an increase of the distortion due to a coil (i.e., a LPF coil) within the low-pass filter; and degradation or deterioration on the damping factor thereof.
For reducing such the adverse effects, a negative feedback can be considered to be effective, being taken from an output terminal of the low-pass filter (e.g., the junction point between the low-pass filter and the load (such as, the speaker)). However, if the low-pass filter is a filter of the second order of LC or higher, then the phase is delayed by 180° even only in this portion, and therefore it is impossible to adopt such the negative feedback, in general, because the negative feedback under such the condition results in an oscillation.